What I know about buying a mattress I've learned from the best: Insiders who have retired from the sleep product industry.
It is impossible to compare by make and model from one store to another. No wonder mattress retailers are so quick to promise that if you can find the same mattress at a lower price, they'll give it to you for free. They know that will never happen because no other store carries that exact mattress.
Shop by level
Each mattress manufacturer makes "levels" of mattresses for the stores that sell their brand: Very cheap, decent cheap, good and best. That's not what they call them, but you can tell by the pricing within each manufacturer's line of products.
A very cheap mattress is about 10 percent materials (foam, steel, padding) and 90 percent air. A mid-level mattress is about 40 percent materials and so on. The more material, the better the product and the higher the price. A high-quality mattress will be 90 percent materials, which means it will be the heaviest of those you try to lift.
The heavier the person(s) who will be sleeping on the mattress, the heavier you want the mattress to be. You spend 30 percent of your life on your mattress. It makes a lot of sense to spend twice the price of a cheap mattress to have it last four times as long.
No pillow top
A pillow top mattress is a normal mattress with a layer of extra padding on top. The pillow top will wear out and flatten down long before the actual mattress begins to show a dent. Buy a great mattress pad instead. Then throw it away when it mashes down and buy a new one. Using this technique, you will have a mattress that lasts many years longer.
Take a nap
Once you've narrowed your choices to two or three, get comfortable for at least 15 minutes on each of the beds you are considering. Do not be shy about taking a short nap on the beds you are considering. Do not make a decision based on the way it looks or by sitting on it for 30 seconds.
At the very least, you should get free delivery and removal of your old mattress as part of the deal you make in a mattress or department store. And at the most? You could get a price reduction, a mattress cover, sheets and pillows too.
Remember the first rule of negotiating: Always ask for more than you're willing to accept. That way, when your opponent comes back and agrees to only some of your counter offer, both of you have the opportunity to come out winners.
Mary invites questions at firstname.lastname@example.org, or c/o Everyday Cheapskate, P.O. Box 2099, Cypress, CA 90630.